Within the framework of the deployment of wireless networks, the design of antennas is confronted with a particular problem due to the way in which the various frequencies are allocated to these networks. Thus, in the case of domestic wireless networks according to the IEEE802.11b and IEEE802.11a standards, a frequency band at 2.4 GHz and two disjoint frequency bands around 5 GHz have been allocated for the deployment of wireless networks according to these standards. In this case, the spectrum to be covered is therefore composed of three disjoint sub-bands. The same phenomenon is encountered in respect of antennas that have to operate on two disjoint frequency bands such as GSM, GPRS, UMTS antennas, etc.
Moreover, several standards currently exist for wireless networks and the products currently used in these networks follow one or other of these standards. Therefore, it is necessary to have antennas able to operate on disjoint frequency bands.
To remedy this type of problem, the most obvious solution consists in using a wideband antenna which at one and the same time covers all the frequency bands required. It is apparent however that the use of a wideband antenna is not desirable for such coverage. Specifically, in this case, the band covered is very large relative to the necessary band, presenting various drawbacks. Thus, the use of a wideband antenna may encourage the degradation of the performance of the receiver on account of the presence of jammers operating in the band covered by the antenna and, in particular, the band not allocated in application thereof. Moreover, it requires more severe filtering constraints at the level of the transmitter in order to comply with the out-of-band transmission power masks. This generally entails a high cost in respect of the design of the antenna and of the equipment that makes it operate.
Another solution consists in using an antenna operating on a lower frequency band but capable of frequency agility so as to switch over to one or other of the bands. In this case, it is necessary to use one or more active elements to modify the operating frequency of the resonant antenna. However, such a structure is more complex and hence more expensive. Moreover, antennas of this type do not make it possible to cover distantly separated frequency bands.